Track properties window lifetime via QPointer

Previously the properties window would sometimes not receive
a closeEvent, leaving a dangling pointer in OBSBasicMain (and resulting
in a crash when trying to open a new properties window or closing
the application)

obs/window-basic-main.cpp

@@ -979,8 +979,3 @@ config_t OBSBasic::Config() const
 {
 	return basicConfig;
 }
-
-void OBSBasic::UnloadProperties()
-{
-	properties = nullptr;
-}